The invention relates to ground-engaging wheels for vehicles, of the kind comprising a central hub rotatable about a wheel axis, a wheel rim encircling the hub and spaced therefrom, and a plurality of similar circumferentially spaced spoke assemblies connected between the hub and wheel rim, each spoke assembly being resiliently deformable from a datum position so as to permit relative displacement between the hub and rim. Wheels of this kind are described, for example, in my earlier British Patent Specifications Nos. 2,059,887 and 2,109,752.
The wheels disclosed in these earlier specifications are described as preferably having a flexible wheel rim so that the rim deforms under load, such deformation being permitted, and resiliently opposed, by the spoke assemblies. Thus, when the vehicle is subjected to the normal vertically downward load, the lower part of the wheel rim flattens so as to increase the length of rim in close engagement with the ground so as to provide more effective driving engagement between the wheels and the ground, and absorption of unevenness in the ground surface. Furthermore, when the wheel engages an obstacle such as a step in the course of forward motion, the deformation of the wheel rim reduces the upward component of movement of the wheel axis as the wheel rises over the obstacle, thus smoothing the ride over the obstacle and reducing the effort required.
Wheels of this kind find particular application to invalid wheelchairs although they are not, of course, restricted to such use.
It has been found, however, that while the deformation of the wheel rim is extremely useful in enhancing the step-climbing ability of a wheelchair, the deformation of the wheel rim when travelling over substantially smooth ground can give rise to disadvantages. For example, the large tread pattern provided by the deformed wheel is unnecessary for normal indoor use and can lead to unacceptable scuffing damage to the floor surface when steering around a curved path, particularly where the floor surface is carpeting. The present invention therefore sets out to provide a ground engaging wheel of the kind first referred to which retains the step mounting and obstacle mounting advantages of the wheel while avoiding the disadvantages of its use on comparatively smooth ground. This is achieved by constructing the wheel so that while deformation or displacement of the wheel rim relatively to the hub takes place when an obstacle is encountered, no significant displacement or deformation takes place during normal rolling of the wheel on smooth ground.